Subject: repair report ETX-90
Sent: Monday, December 11, 2006 05:25:18
From: a. toonen (a.toonen@home.nl)
Repairing a Meade ETX90 with Motor Unit Failure.
Downloaded printlayout of the panel PCB via Mike's website. Found
reference to US Patent Office for the Meade ETX-90 patent. Downloaded
and studied the patent to understand how the ETX-90 works (I2C-bus is
used for communication between Autostar and both motor units.)
Tried internal batteries. Tried external power supply. Tried #497
Autostar. Tried #494 Autostar. Tried hand controller. All gave the same
result: MUF.
Internal inspection: encoders and fototransistors are clean.
Visual inspection of all PCB's showed no burned components.
Checked supply voltages on panel PCB and both motor units PCB's: all OK.
Exchanged the order of the motor unit cables on the panel PCB. Still
MUF. Both motor units defective?
Disconnected both cables: MUF. This is very strange, looks like bad
contacts.
Checked all interconnections from panel PCB to Autostar connector, HBX
and AUX connector. All OK.
Checked interconnections from panel PCB to both motor units. BINGO:
ground line was not connected. So both motor unit got no power!
Why was the supply voltage check on both motor units (see above) OK?
Because I used a common ground on the panel PCB and the +5 and +12 were
OK.
Problem must be in the panel PCB. Removed (horizontal) motor unit PCB
and motor to get access to the panel PCB. Found out (Mike's website)
that this panel PCB is snap-fit and glued, so cannot be removed.
Used the downloaded printlayout to check at the back of the print where
the connection was broken. Located two "isolated islands", one connected
to the panel connectors and the other to the motor units.
Soldered one tiny isolated copper wire to both islands and that was it.
ETX-90 worked perfectly again.
Conclusion: as I have no proper access to the component side of the
panel print, it is not possible to find the real cause of the bad
connection (bad soldering joint, broken PCB-trace etc.)
Next time I will check supply voltages on satellite PCB's with two
probes on the PCB (and not the common ground on an easy accessible
place).
Arnold W. Toonen

Subject: repair report ETX-90 - Repairing a Meade ETX90 with Motor Unit Failure
Sent: Saturday, December 16, 2006 11:05:50
From: Niall J. Saunders (niall@njs101.com)
Welcome to the wonderful world of repairs - and to the tormenting
problem of missing 'earths' or 'grounds'.
If you do any kind on electronic 'investigation', using meters and
oscilloscopes, you become so used to hooking the 'black lead' somewhere
convenient, and then probing around the circuit with the 'red lead'.
However, this tunnel-vision approach means that we often forget to even
consider that the zero volt line, track, plane, or whatever, may itself
be 'broken'. A very high percentage of my most insidious repairs will
usually have a broken ground circuit, and I always kick myself for not
checking them first.
It is an easy enough task to do - just don't use the 'black lead' as
your common reference on the DMM or Scope - instead, use the red lead
onto the supply line, and probe around the circuit with the black lead
instead. Now, instead of looking for 'supply level' type voltage
indications, you are looking for 0 Volt readings (when you are checking
for broken, or missing, earths). Anything other than 0 V, and you can
start being very suspicious !!
In the ETX (and LX) scopes there is a very common point of failure that
manifests itself as a 'missing / broken ground' fault. This occurs when
the scope has been used on an external power source, with a 5.5mm/2.5mm
power plug being inserted into the Aux Panel. When this is done, a
switch contact inside the power socket is opened (by the outer barrel of
the inserted plug). This switch opens in order to disconnect the
internal batteries (if fitted) - to prevent the external power supply
from 'charging' any internal batteries - which would have almost certain
disastrous results. However, I have come across several scopes now that
no longer work on internal batteries - because the socket used by Meade
fails, and the switch contact does not close properly when the external
power plug is removed.
As you have discovered - this is a major problem, simply because of the
difficulty in removing the auxiliary control panel to get access to the
power socket. The socket itself would cost mere cents to replace, but
you will have a major struggle ahead of you just trying to get the panel
out.
It seems that, in your case, the problem was not associated with the
power socket, but was a poor ('dry') solder joint elsewhere on the PCB -
hopefully you were 'lucky' in being able to get to it and cure it.
By the way, another problem that can cause MUFs - Motor Unit Faults - is
the HBX connectors themselves. If you get any kind of tarnishing, or
corrosion, on the gold wires that form the pins of the HBX socket, you
can end up with one, or more, axes that do not respond to Autostar
control signals. I have experienced this, and find that one cure is
simply to insert and remove the HBX plugs many, many, many times - until
the contacts wipe themselves clean once more. Sometimes a 'miracle cure'
can be observed just by swapping the curly-cord lead 'end-for-end' - but
there is no guarantee that this will work.
I have also had an Autostar 'fry itself', on my LX90, as a result of
pins in the (unused) Aux connector socket(s) having been bent (long
before I bought the scope - because I have never had to use any of the
Aux sockets on my scopes). Repairing this problem was also a case of
several hours of work with dental picks and a big magnifying glass !!
(The Autostar remains dead, until such time as I can find another 'dead'
one to donate suitable components!!)
Hope this helps.
Cheers,
Niall Saunders
Clinterty Observatories
Aberdeen, SCOTLAND